Functions Error: Impacting Galleries

You can try to add the condition $query->is_main_query() in your if-sentences so it won’t affect the gallery queries:

// Posts per page
add_filter('parse_query', 'wpq_parse_query');
function wpq_parse_query($query)
{
    if(!$query->is_main_query())
    {
        return $query;
    }

    if($query->is_archive())
    {
        $query->query_vars['posts_per_page'] = get_option('to_count_archives', 1);
    }
    if($query->is_home())
    {
        $query->query_vars['posts_per_page'] = get_option('to_count_home', 1);
    }
    if($query->is_search())
    {
        $query->query_vars['posts_per_page'] = get_option('to_count_search', 1);
    } 
    return $query;    
}